Workers with Miller Environmental Group collect absorbent material used to soak up oil from a spill on the surface of the water near Ronald Reagan National Airport on Feb. 8 in Arlington. (Kate Patterson/for The Washington Post)

About 20 Canada geese were released Monday after a rescue group helped clean them following an oil spill last month in the Potomac River.

The Delaware-based Tri-State Bird Rescue and Research group retrieved about 60 waterfowl that were taken out of the Roaches Run Waterfowl Sanctuary, which feeds into the Potomac, three weeks ago. The Coast Guard reported that 29 of those birds have died.

Dominion Virginia Power acknowledged that about 13,500 gallons of oil spilled Jan. 25 from its Crystal City substation. The utility said it cleaned up 95 percent of the spill and said the other 5 percent remained in the substation’s machinery. But a Coast Guard analysis found that the oil that leaked into Roaches Run Waterfowl Sanctuary and the Potomac River a week later matched the Dominion oil.

Dominion accepted responsibility for the leak once the tests were completed.

Last week, the Coast Guard said that no additional sheening had been observed in the refuge or on the river and that no additional harm to wildlife had been reported.

On Monday, a batch of geese — about 21 birds — that were saved from the water were returned, according to Tri-State Bird Rescue and Research. And on Friday, a duck was put back in the river, officials said. Ten more geese are still being cared for by the bird rescue group.

Over the past few weeks, rescuers said the birds have gone through a washing and cleaning process and have been fed by caretakers. Some had skin irritation and feather loss, according to Lisa Smith, executive director of the bird rescue group.

Officials said the birds are now in good shape and ready to be released. The ones that are still being cared for are being watched to make sure their feathers are coming in properly, Smith said.